Known plants for manufacturing cement clinker may comprise several cyclone stages in succession through which pass hot gases such as exit gases from, e.g., a calciner and/or the kiln. Raw meal is supplied to the first cyclone stage and passed countercurrently to the hot exit gases down through a cyclone preheater to the calciner from which the precalcined raw meal is fed via a cyclone separator to the kiln. Combustion air for the calciner may, e.g., be supplied from a clinker cooler coupled after a kiln.
Preheating of the raw material in the preheater is obtained by contact. Thus direct heat transfer occurs between the raw material and the exit gases.
If a cyclone preheater as described above--i.e. wherein direct contact occurs between the raw material and the exit gases--is used for preheating raw material having volatile, combustible components, a risk of explosion will be imminent when the combustible components have been expelled from the raw material and are absorbed in the exit gases. Additionally, the heating performance of the expelled gases is lost.
The present invention relates to an apparatus for use with a kiln for manufacturing cement clinker comprising at least one multi-stage preheater for passage of cement raw material therethrough, the at least one multi-stage preheater having at least one cyclone stage heated by exit gases, means associated with at least one preheater stage for separating the cement raw material from said exit gases, and means for transferring heat indirectly from said exit gases to the cement raw material.
In a preferred embodiment, the present invention relates to an apparatus for manufacturing cement clinker from cement raw materials having volatile, combustible components, comprising a kiln provided with a clinker cooler and a multi-stage preheater for passage of the cement raw material therethrough, the preheater being heated by hot exit gases from at least one of the kiln and the clinker cooler and having at least one cyclone preheater stage, means provided in at least one stage of the multi-stage preheater for separating the cement raw material from the exit gases, and means for transferring heat indirectly from the exit gases to the cement raw material such that the volatile, combustible components may be expelled from the cement raw material without explosion of the volatile, combustible components.
The present invention also relates to a plant for manufacturing cement clinker from cement raw material characterized in that at least the first stage of the preheater comprises a heat exchanger having a raw material section, a separate hot gas section, and means for enabling indirect heat transfer between the hot gas section and the raw material section.
The hot gases typically will be the exit gases from the kiln and/or calciner. Alternatively, they may be hot air from the cooler depending on the design of the plant.
The present invention also relates to a method of manufacturing cement clinker from cement raw material including the steps of heating the raw material by passing it in a first stream through a heat exchanger forming at least the first stage of a multi-stage preheater and indirectly transferring heat from a separate stream of hot gases fed through the heat exchanger.
The present invention also relates to a method for use with a kiln for manufacturing cement clinker in an apparatus including at least one multi-stage preheater for passage of cement raw material therethrough, the at least one multi-stage preheater having at least one cyclone stage heated by exit gases, comprising maintaining the cement raw material separate from the exit gases, and transferring heat indirectly from the exit gases to the cement raw material.
Since the hot gases are kept separate from the raw material during the expulsion of any volatile, combustible components of the raw material, the hot gases do not become explosive. In addition, the hot gases do not contain components which may cause air pollution. Furthermore, the volatile, combustible components expelled from the raw material can be utilized in the kiln or the calciner as additional fuel instead of being wasted.
In the preferred embodiment of the present invention, the heat exchanger may advantageously have two chambers, hot gases flowing through one of the chambers, and raw material flowing through the other. The chambers are separated by a partition through which so called "heat-pipes" project into both chambers so as to ensure heat transfer from the hot gas chamber to the raw material chamber.
Such heat-pipes are of the type known to those skilled in the art. In their simplest form, the heat-pipes may be lengths of pipes closed at both ends containing a fluid capable of evaporating and condensing. The fluid absorbs heat and evaporates at the relatively hot end of each pipe and then condenses thereby releasing heat at the relatively cold end. The condensed fluid is then returned to the hot end of the pipe. For this purpose the pipes may be slightly inclined with the relatively hot ends being in the lowermost position.
The hot gas chamber of the heat exchanger may contain a granular, heat-resistant material for the formation of an expanded fluidized-bed made fluid by means of the hot gases passing therethrough. In this fashion, the heat transfer from the hot gases to the heat-pipes is thereby improved.
As a result of the raw material flowing downwards in the raw material chamber of the heat exchanger, fluidized bed-like conditions are already present in this chamber, which gives effective heat transfer from the heat-pipes to the raw material.